Field
Exemplary embodiments of the present disclosure relate to porous ceramic articles and a method of making the same. Exemplary embodiments of the present disclosure relate to porous ceramic articles having microstructure including sinter bonded or reaction bonded large pre-reacted particles and pore network structure and a method of making porous ceramic articles using pre-reacted particles.
Discussion of the Background
Cordierite, silicon carbide, and aluminum titanate-based honeycombs have been widely used for a variety of applications including catalytic substrates and filters for diesel and gasoline engine exhaust after treatment.
To meet increasingly stringent emission regulations for light and heavy duty vehicles, the substrate and filter materials have to be highly porous to allow gas flow through the walls without restricting the engine power, have to show high filtration efficiency for emitted particles, and, at the same time, are expected to demonstrate low back pressure. The substrates and filters also have to be able to withstand the erosive/corrosive exhaust environment and bear thermal shock during rapid heating and cooling. Regulation of CO2 emission and raising fuel cost drive miniaturization and integrated functionality in the exhaust gas after-treatment system. It may be desirable to reduce the number of components in the after-treatment system, decrease their size and implement multi-functionality of the different components. For example, integrating de-NOx catalyst and diesel oxidation catalyst (DOC) into diesel particulate filters may be desired. To reach high de-NOx efficiency, rather high loading of de-NOx catalyst is required together with a high catalyst activity at low temperature, such as can be found for Cu-zeolites. Trends and Original Equipment Manufacturers (OEMs) desires may drive zeolite catalyst loading to high levels of 200 g/l. In order to meet this loading target and preserve low pressure drop, the filter substrate may need high porosity and large pore size, for example, around 60% porosity with a median pore size of 18 μm or larger.
High porosity and large pore size that enables high de-NOx efficiency are expected to not degrade the particulate filtration efficiency. They should also not decrease the thermo-mechanical properties of the filter. Cordierite and aluminum titanate may both have low thermal expansion and are therefore suited for applications where high thermal shock resistance is required. Both materials show anisotropy in their thermal expansion with different crystallographic directions exhibiting positive and negative expansion. Due to the anisotropy in thermal expansion, mismatch strains build up between grains with different crystallographic orientation; such strains can lead to microcracking. Polycrystalline cordierite or aluminum titanate ceramics may undergo extensive microcracking during thermal cycling. Microcracks open during cooling and close, sometimes even heal during heating. This creates a hysteresis response to thermal cycling with differences between heating and cooling that can be attributed to the reversible microcrack formation and closure. As a consequence of microcracking, the overall coefficient of thermal expansion (CTE) of the ceramics may be lower than the crystallographic average CTE.
On first look, microcracking may seem beneficial; the thermal shock resistance of the material, which is proportional to the material's strength and inversely proportional to its elastic modulus and thermal expansion, is expected to be improved by microcracking. However, the material strength also decreases with increasing microcrack density. Microcrack densities in cordierite remain rather low, due to the small difference in crystallographic thermal expansion and large grain (domain) sizes required to reach the stress threshold for microcracking. As a result of a much larger anisotropy in crystallographic expansion, microcrack densities in aluminum titanate-based materials are much higher and strongly influence the ceramic article's strength.
Porous cordierite and aluminum titanate based honeycomb ceramic articles with low thermal expansion, high porosity, low Young's modulus and high strength are utilized as high-performance automotive catalytic converter substrates and diesel particulate filters. For cordierite products, raw materials such as alumina, talc, clay, magnesia, alumina and silica powders may be mixed with organic binders and pore formers. For aluminum titanate composite products, raw materials such as alumina, titania powders and raw materials for forming the “filler” phase, for example strontium oxide, alumina, silica to form feldspar (strontium aluminum silicate feldspar or “SAS”), may be mixed with organic binders, pore formers and water to form a plastic mixture. The plastic mixture may be extruded or otherwise shaped into a green body of desired shape, for example, a honeycomb, trough log or disk filter, dried, and then fired to temperatures between 1350° C. and 1450° C., depending on the raw material combination. During the drying and firing process, the raw material particles react, and form, via various intermediates, the final crystalline cordierite or alumina titanate composite. The shaped green part transforms upon firing into a solid, durable porous ceramic article. Other substrate and filter honeycomb materials or mixtures of materials that upon high temperature treatments react to form oxide or non-oxide ceramics, may include metals, intermetallics, mullite, alumina (Al2O3), zircon, alkali and alkaline-earth alumino-silicates, spinels, perovskites, zirconia, ceria, silicon nitride (Si3N4), silicon aluminum oxynitride (SiAlON), and zeolites.
Diesel particulate filters (DPF) and gasoline particulate filters (GPF) may be obtained from a honeycomb porous ceramic by plugging channels in a checkerboard pattern on one end and plugging the remaining channels at the other end to form a filter with inlet and outlet channels. The exhaust gas flows into the open inlet channels, through the wall of the honeycomb (through-wall flow) because the inlet channels are plugged at the other end and out of the outlet channels, which are plugged at the inlet end. During exhaust gas passage through the porous honeycomb wall, small particulates from the exhaust gas are deposited on the pore surface or as the soot layer on the wall surface, thus providing filtering of the exhaust gas. The soot cake of deposited particulates may be periodically burned in a regeneration cycle or continuously during passive regeneration so that the DPF or GPF has a lifetime similar to that of the vehicle. Alternative filter designs may be used, such as radial trough filters or radial disk filters, which compared to the honeycomb design with its long, narrow gas flow channels may show wider gas flow channels and a stronger radial component for the gas flow, but share the same particulate filtering of the gas when passing through the thin porous ceramic wall and offer the same opportunity for de-NOx functionality with incorporation of a suited catalyst in the wall-porosity and/or on the channel walls.
Tightening of exhaust gas regulations may call for higher particulate filtration efficiency, particularly for small particle size, and for higher NOx filtration efficiency, not only in the currently established test cycles, but also in continuous real-world driving. CO2 regulations may call for use of less fuel and OEMs demand lower pressure drops, both at improved thermal shock resistance and extended lifetime of the porous ceramic honeycomb substrate. To meet these demands, substrates and filters with higher porosity, larger pore size, with thinner honeycomb walls than currently in use may be needed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention as claimed and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.